Chapter 3 Deadlocks 3.1 Resource 3.2 Introduction to deadlocks 3.3 The ostrich algorithm 3.4 Deadlock detection and recovery 3.5 Deadlock avoidance 3.6 Deadlock prevention 3.7 Other issues
Resources • Examples of computer resources – printers – tape drives – tables • Processes need access to resources in reasonable order • Suppose a process holds resource A and requests resource B – at same time another process holds B and requests A – both are blocked and remain so 2
Resources (1) • Deadlocks occur when ... – processes are granted exclusive access to devices – we refer to these devices generally as resources • Preemptable resources – can be taken away from a process with no ill effects • Nonpreemptable resources – will cause the process to fail if taken away 3
Resources (2) • Sequence of events required to use a resource 1. request the resource 2. use the resource 3. release the resource • Must wait if request is denied – requesting process may be blocked – may fail with error code 4
Introduction to Deadlocks • Formal definition : A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause • Usually the event is release of a currently held resource • None of the processes can ... – run – release resources – be awakened 5
Four Conditions for Deadlock 1. Mutual exclusion condition • each resource assigned to 1 process or is available 2. Hold and wait condition • process holding resources can request additional 3. No preemption condition • previously granted resources cannot forcibly taken away 4. Circular wait condition • must be a circular chain of 2 or more processes • each is waiting for resource held by next member of the chain 6
Deadlock Modeling (1) A S D T U R B C (a) (b) (c) • Modeled with directed graphs (a) resource R assigned to process A (b) process B is requesting/waiting for resource S (c) process C and D are in deadlock over resources T and U 7
Deadlock Modeling (2) Strategies for dealing with Deadlocks 1. just ignore the problem altogether • Ostrich Algorithm 2. detection and recovery 3. dynamic avoidance • careful resource allocation 4. prevention • negating one of the four necessary conditions 8
Deadlock Modeling (3) How deadlock occurs 9
Deadlock Modeling (4) How deadlock can be avoided 10
The Ostrich Algorithm • Pretend there is no problem • Reasonable if – deadlocks occur very rarely – cost of prevention is high • UNIX and Windows takes this approach • It is a trade off between – convenience – correctness 11
Detection with One Resource of Each Type (1) R A B E C S D T E D R F U V U V G G W (a) (b) • Note the resource ownership and requests • A cycle can be found within the graph, denoting deadlock 12
Detection with One Resource of Each Type (2) Resources in existence� Resources available� (E 1 , E 2 , E 3 , …, E m ) (A 1 , A 2 , A 3 , …, A m ) Current allocation matrix Request matrix C 11 C 12 C 13 C 1m R 11 R 12 R 13 R 1m C 21 C 22 C 23 C 2m R 21 R 22 R 23 R 2m C n1 C n2 C n3 C nm R n1 R n2 R n3 R nm Row n is current allocation� Row 2 is what process 2 needs to process n Data structures needed by deadlock detection algorithm For vectors A and B (m), A ≤ B iff Ai ≤ Bi for 1 ≤ i ≤ m 13
Detection with One Resource of Each Type (3) s s e e s s s s v v r m r m i e i e s s r r o o d r n d r n e e R R n n e t e t t a t a p o D p o D c c a l a l P S C P S C T T E = ( 4 2 3 1 ) A = ( 2 1 0 0 ) Current allocation matrix Request matrix 0� 0� 1� 0� 2� 0� 0� 1� C = 2� 0� 0� 1� R = 1� 0� 1� 0� 0 1 2 0 2 1 0 0 An example for the deadlock detection algorithm 1. R2 : (2 1 0 0) ⇒ A = (2 2 2 0) 2. R1 : (1 0 1 0) ⇒ A = (4 2 2 1) 3. R0 : (2 0 0 1) ⇒ A = (4 2 3 1) 14
Recovery from Deadlock (1) • Recovery through preemption – take a resource from some other process – depends on nature of the resource • Recovery through rollback – checkpoint a process periodically – use this saved state – restart the process if it is found deadlocked 15
Recovery from Deadlock (2) • Recovery through killing processes – crudest but simplest way to break a deadlock – kill one of the processes in the deadlock cycle – the other processes get its resources – choose process that can be rerun from the beginning 16
✁ ✁ ✁ � � � � Deadlock Avoidance Resource Trajectories B u (Both processes� finished) Printer I 8 I 7 I 6 t I 5 Plotter r s A p q I 1 I 2 I 3 I 4 Printer Plotter Two process resource trajectories 17
Safe and Unsafe States (1) Has Max Has Max Has Max Has Max Has Max A 3 9 A 3 9 A 3 9 A 3 9 A 3 9 B 2 4 B 4 4 B 0 –� B 0 –� B 0 –� C 2 7 C 2 7 C 2 7 C 7 7 C 0 –� Free: 3� Free: 1� Free: 5� Free: 0� Free: 7� (a) (b) (c) (d) (e) (b) finished, (c) finished, A can be finished Demonstration that the state in (a) is safe 1. Not deadlocked 2. ∃ scheduling over each process can request MAX 18
Safe and Unsafe States (2) Has Max Has Max Has Max Has Max A 3 9 A 4 9 A 4 9 A 4 9 B 2 4 B 2 4 B 4 4 B — — C 2 7 C 2 7 C 2 7 C 2 7 Free: 3 Free: 2 Free: 0 Free: 4 (a) (b) (c) (d) Demonstration that the sate in b is not safe deadlock ⊆ unsafe (a) Give A one more (d) A: 5 needed, C: 5 needed, only 4 available 19
The Banker’s Algorithm for a Single Resource • Three resource allocation states (a) safe – Any order (b) safe – C(4), B(5), D(9), one possibility (c) unsafe – None can request MAX 20
Banker’s Algorithm for Multiple Resources Example of banker’s algorithm with multiple resources • E = Exiting • P = Possessed • A = Available 21
Deadlock Prevention Attacking the Mutual Exclusion Condition • Some devices (such as printer) can be spooled – only the printer daemon uses printer resource – thus deadlock for printer eliminated • Not all devices can be spooled (e.g. process table) • Principle: – avoid assigning resource when not absolutely necessary – as few processes as possible actually claim the resource 22
Attacking the Hold and Wait Condition • Require processes to request resources before starting – a process never has to wait for what it needs • Problems – may not know required resources at start of run – also ties up resources other processes could be using • Variation: – process must give up all resources – then request all immediately needed 23
Attacking the No Preemption Condition • This is not a viable option • Consider a process given the printer – halfway through its job – now forcibly take away printer – !!?? 24
Attacking the Circular Wait Condition (1) Left: Normally ordered resources Right: A resource graph 25
✄ ✄ ✄ ✄ ✄ ✄ ✄ ✄ ✄ ✄ ✄ � ✄ ✄ ✄ ✄ ✄ ✄ ✄ ✄ ✄ � ✄ � � � � Summary of approaches to deadlock prevention �✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁� Condition Approach �✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁� Mutual exclusion Spool everything �✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁� Hold and wait Request all resources initially �✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁� No preemption Take resources away �✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁� Circular wait Order resources numerically �✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁�✂�✁�✁�✁�✁�✁�✁�✁� 26
Other Issues Two-Phase Locking • Phase One – process tries to lock all records it needs, one at a time – if needed record found locked, start over – (no real work done in phase one) • If phase one succeeds, it starts second phase, – performing updates – releasing locks • Note similarity to requesting all resources at once • Algorithm works where programmer can arrange – program can be stopped, restarted (in this way) 27
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